Spark Plug for RPM Sensing

Hello everyone,

Thank you very much for taking the time to read this.

I recently came across a very cool blog post where someone used a spark plug to find RPM by wrapping the spark plug wire a few times and then taking resulting inductive current to drive transistor, which then pulses a Schmit Trigger, and then goes to Pin 2 on Arduino.

I am looking to recreate this, though am not familiar with Schmit Triggers? Also, I think that the system I'll be using has higher voltage spark plugs, will this change the spec for the diodes and resistors etc?

Any guidance would be greatly appreciated.

Thank you very much!!!

I guess the thing that I'm having the hardest time understanding is the conversion of the AC to DC current. The diodes are acting as a rectifier. Does the capacitor have to charge to a certain voltage before it can it will activate the transistor?

Any help is greatly appreciated, thank you!

can you get any sort of reading or action now ?

the coil you are making will create the voltage based on the voltage in your wires.
the first diode is to eleminate a reverse condition.
the voltage divider is to create the voltage for the cap. MAKE SURE you do not have more voltage than the cap. it looks like the OP is using a high voltage cap, not a electrolytic.
the cap is used for storage and is constantly drained to ground with the 10k resistor

there is a 5 volt limit by the zerner diode.
this is on the transistor base.

as a test, I would just use an LED from the power, with a resistor, through the transistor and watch it glow with RPM.

if you have that, then the rest is easy. it appears your hard part is to get a variable signal.